The present invention relates to heating a metal product advancing continuously in a protective atmosphere or in a vacuum, by electromagnetic induction.
The use of electromagnetic induction heating devices for heat treatment in a protective atmosphere as implemented in hot coating lines (for galvanizing, aluminizing, etc.) or continuous heat treatment lines (for example annealing lines) has been known for a long time.
Generally, two types of induction heating devices are used for these plants, namely one with a longitudinal flux (the inductor consists of several turns through which current passes and which surround the advancing product so as to create a magnetic flux in the direction of advance of the product) and one with a transverse flux (the turns of the inductor are placed in a plane parallel to the surface of the product such that the magnetic flux is perpendicular to this surface).
The use of one or other of these devices is chosen depending on the type of product to be treated. Thus induction with a longitudinal flux is generally suitable for magnetic products whose temperature is required to be in the vicinity of 750xc2x0 C. (Curie point) whilst induction with a transverse flux is more suitable for heating at a higher temperature, especially for non-magnetic products.
Furthermore, when an uncoated product is involved, which must be treated at a temperature where there is a risk of it oxidizing, the induction heating must be carried out under a protective atmosphere, such as a mixture of hydrogen and nitrogen. The result of this is that the heating device comprises, inter alia, a chamber through which the product to be treated passes and which must be completely airtight so as to prevent any oxidation of the product.
In the known applications of this type of induction heating, induction heating devices are used in a protective atmosphere, in which the heating device is placed directly inside the sealed chamber. The seal is therefore then made all around the induction means, which presents many drawbacks, especially during operations of maintaining these inductors, which require the chamber to be dismantled, or for sealing the passages for the inflow and return of electric current supplying the inductors and for those of the water circuits cooling the latter.
To solve this problem, applications as described, for example, in European patent application published under No. 0 822 733 are known, which uses a heating device comprising induction means and a gastight chamber placed around the advancing product, between the latter and the inductors, the said chamber lying in the direction of advance of the product, upstream and downstream of the inductors.
In this European patent application, the sealed chamber, in its central part enclosed by the actual induction means, consists of a sleeve comprising one or more layers of thermally and electrically insulating fabric and coated with a gastight film able to withstand a temperature of at least 100xc2x0 C. but never exceeding 750xc2x0 C. (device operating by induction with a longitudinal flux).
On the other hand, when heating a product to a temperature greater than 750xc2x0 C. is involved (for example for the bright annealing of stainless steel) using an induction heating device with a transverse or longitudinal flux, the sleeve disclosed in this document is no longer effective at such temperatures, which risks damaging the inductors which typically should not operate at more than 100xc2x0 C.
The present invention therefore aims to overcome such drawbacks by providing an electrically insulating (that is to say transparent to the magnetic flux) gastight and vacuum-tight chamber coupled to a thermally insulating shield which makes it possible to protect the induction means and any magnetic circuit of the electromagnetic induction heating device, and this regardless of how the latter operates (longitudinal flux or transverse flux).
According to the invention, this gastight and vacuum-tight chamber is placed around the advancing product to be heated, between the latter and the induction heating means.
To this end, the gastight and vacuum-tight chamber according to the invention intended to be used in a device for heating a product advancing inside the said chamber by electromagnetic induction is characterized in that it comprises, at least in its central part surrounded by the said induction heating means, a sheath made of an electrically insulating gastight and vacuum-tight material, the inner faces of the said sheath being protected by a heat shield consisting of a matrix of tiles made of a thermally insulating material and of a plurality of tubes cooled by the flow of a fluid, the latter being trapped in the said matrix of tiles.
The role of the heat shield is to ensure a temperature close to 100xc2x0 C. for the material forming the sheath, while the product to be heated is at a temperature of about 1200xc2x0 C.
Other characteristics and advantages of the present invention will emerge from the description given below with reference to the appended drawings which illustrate an embodiment thereof which is devoid of any limiting character. In the figures: